1. Field of the Invention
This invention discloses a low-cost continuous wave laser DPIV (Digital Particle Image Velocimetry). The photographs of a particle-seeded flow field, illuminated by a laser sheet, taken by the CCD (Charge Couple Device) camera with different time intervals between exposures and then processed by a computer, can be used to detect particles"" locations, displacements and their moving loci in the flow field, and accordingly determine the particles"" velocities and the 2-D (two-dimensional) spatial distribution of instantaneous velocity in the flow field.
2. Background of the Invention
In early studies of fluid mechanics, such an intrusive measurer as Hot Wire Anemometer or Pitot Tube was used in a flow field to measure velocity data at a specific point, and Flow Visualization Techniques to observe the qualitative data like geometric distributions of streamlines, shapes of flow fields, etc., and thus to learn the properties of the flow fields.
With the new and flourishing development in Optics, Electronics, Image processing, Laser technology, and digital computers, LDV (Laser Doppler Velocimetry) has been widely applied in measuring the single-point velocity in a flow field. Such xe2x80x9cnon-intrusive measurementxe2x80x9d will both maintain the integrity of a flow field and meanwhile improve the accuracy of the velocity and Turbulence measurement. However, Single-point Measurement Technique can only obtain data at only one single point without immediate and complete velocity distribution information of an entire plane. Thanks to the efforts of forerunners, in late 70""s, PSV (Particle Streak Velocimetry) was first reported by Simpkins, P.G. et al. (J. of Fluid Mech. Vol.89, pp.665-671, 1978). A transitory Mie scattering photo of a flow field seeded with micro particles was taken by a CCD (Charge Couple Device), via scanning of laser or other resources of illumination, to analyze (either with the film or the picture) the moving loci of particles in the flow field, and accordingly to make clear the velocity magnitude and direction of particles in a 2-D plane and the spatial distributions of the velocity in the flow field. Equipment for such a velocity measuring method via loci was simple to manipulate, and velocity of particles could be easily figured out through manual analyses. But to complicated images of more and speedy particles, it failed to process and thus determine the exact velocity direction of a flow field.
3. Description of the Prior Art
Once the encoded flashes of different time lapses were used to determine the velocity directions: a single particle might leave line segments of different lengths in the image, and the velocity directions could thus be determined by comparing the segment length and its corresponding code. Similarly, this method was still of little use in processing complicated flow-field images. Late in 80""s, two methods were applied: Young""s Fringe Method reported by Meynart, R. (Applied Optics. Vol.19, #9, pp.1385-1386, 1983) and Auto-correlation Method by He, Z. H. et al. (Experimental Mechanics, pp.117-121, 1984). Since the operations of the former method were too sophisticated, a 2-D auto-correlation analysis was usually adopted instead. Nonetheless, these two methods still had trouble in determining the moving directions of particles, although they were helpful in rather complex images.
A digital particle image velocimetry was thus invented to solve the above-said troubles. Equipment used in this DPIV (Digital Particle Image Velocimetry) method included Pulse Laser, CCD (Charge Couple Device) Camera, Frame Grab, computer, and so on. An image of a flow field illuminated by a high-energy pulse laser was taken by a CCD (Charge Couple Device), and then interfaced to a computer for image processing through a Frame Grabber. For determination of velocity, a Single-Image auto-correlation Analysis can be adopted; for velocity directions, two methods were available: either by exactly controlling the relative time delay of the Double Pulse laser flashes vs. actuations of the camera to determine the sequence of two images and then processing these two images with cross-correlation analysis method, or by using two laser pulses, with one of which delayed and passing through a Raman Tube to change its color, to take two images of different colors, and then using the cross-correlation analysis method to determine the velocity and its direction. It was believed that better measurements could be obtained by such methods, but the outstanding high costs of the Double Pulse high-energy laser, Raman Tube for changing colors, complex delay controller, and high-speed Frame Grabber were not affordable for average research institutes and individuals.
Recently, due to the rapid development of digital color image techniques, different colors of an image could be easily separated. The processing of color separation in an image for cross-correlation analysis is shown in FIG. 1. During the period when the shutter of the CCD camera was activated, a green laser beam from the color alternating device (describe later) first illuminated the particles of a flow field, making the particles scattering green beam-marked as the first time point; then a blue laser beam took turns and thus the second time point marked. As illuminated by the alternative green and blue beams, the same particle appeared in different colors at different positions while moving. The entire period from the beginning of green beam illumination to the end of blue beam illumination was completed during a shutter period. In such a way, a digital color photo (101) was obtained. In order to obtain the velocity distribution, the color photo was divided into small cells called an interrogation window (107), in which black solid marks stood for green beam images (102) and hollow ones for blue beam images (103). Green and blue images could be separated since this kind of digital color photos (101) were comprised of three primary colors, i.e. red, blue and green, and thus could usually be separated by a computer installed with related programs. The purpose of the above-said image color separation (104) was to determine by the mathematic cross-correlation analysis (105) whether two points were left by the same particle respectively at different times and positions in the green and blue images. When the distance between the two points was calculated, then divided by the time time gap between green and blue pulses, one could easily obtain the moving velocity of the particle (106). And just by comparing different color signals in the same image, the direction component of the particulate velocity could be also found. The Color alternating Image Velocity Measurement reported by Jaw, S. -Y. et al. (The 22nd Symposium on Naval Hydrodynamics, Washington D.C., U.S.A., 1998) was developed from the above method, in which a combined beam, produced by an Argon ion laser, could be transformed into a planar laser sheet of two alternating colors by alternatively changing the laser wavelength via an AOM (Acousto-Opto Modulator). Therefore, the images, taken by a CCD (Charge Couple Device) while a flow field was scanned by the alternating dual colors, could be used to determine the direction and size of the flow field and thus determine the 2-D velocity measurement by distinguishing different images taken at different times. The continuous Argon ion laser (1) was inexpensive though it had limitations in range and accuracy of measuring particle velocity. And the combined beam, used in the Alternating Dual Color Image Velocity Measurement, was produced by an Argon ion laser (1), which was a Low-Cost piece of equipment. However, the wavelength of the laser beam had to be transformed through particular AO modulation techniques, and the special AOM (Acousto-Opto Modulator) was difficult in manipulation, controlling and in data acquisition.
The inventor of the Low-Cost Continuous Laser DPIV (Digital Particle Image Velocimetry) has been engaged in observing and measuring flow fields for years and had personally performed all the above said methods. It is assured that the Color Alternating Image applied in image velocity measurement can determine the velocity and direction of particles in a flow field rather accurately and effectively. And even more, velocimitry may also be applied in biological and micro-mechanical technologies in measuring the transmitting speed of particles, non-intrusively. To get rid of the above-said manipulating, controlling and acquiring difficulties, the inventor discarded the particular AO modulation techniques to transform combined beams into color alternating beams. Instead, disclosed is A Low-Cost Continuous-Wave-Laser (CW Laser) Digital Particle Image Velocimetry and equipment conceived by the inventor.
It is therefore an object of the present invention to provide an inexpensive yet effective method and apparatus for determining particulate velocity in a particle flow field.
It is also an object of the present invention to effectively measure particle velocity in a flow field without using AOM and without disturbing the particle flow field when making the measurements.
It is further an object of the present invention to produce a single laser sheet beam alternating between green and blue light using inexpensive equipment, said sheet beam capable of being used to illuminate a particle flow field to determine particulate velocity using cross-correlation analysis.
These and other object can be achieved by the the Low-Cost Continuous Laser DPIV (Digital Particle Image Velocimetry) device, which can transform combined beams into color alternating laser sheets via certain color-alternating equipment to diagnose particle velocity in flow fields. And the color-alternating images taken by the CCD (Charge Couple Device) can be analyzed by a computer, and with the help of the cross-correlation analysis method, to determine the two-dimensional distribution of the magnitude and direction of the flow velocities of particles in a flow field without using AOM.
The Low-Cost Continuous-Wave-Laser (CW Laser) Digital Particle Image Velocimetry can determine the velocity and the direction of a particle in a flow field, the particles"" moving locus, the velocity in a 2-D plane and the velocity distribution in a space via computer processed images taken by the CCD (Charge Couple Device) in particle flow fields.
In the formation of the color alternating laser sheet, two continuous wave monochromatic beams, each of different wavelengths, impinge on a rotating wheel perforated by holes to produce two monochromatic pulsating beams that pulsate in an alternate manner. Then, these two pulsating beams of different color are combined into a single beam to illuminate the particle flow field. This wheel or chopper is rotated at a speed that dictates the length of each pulse and the frequency in which the color alternates in the resulting laser sheet. This information is coupled with a camera that takes exposures, the camera being triggered by the light pulses. The time gap between consecutive images of the particle flow field is found from the speed of the chopper motor. By noting the displacement of a particle in the flow field between the two consecutive images, particle velocity can then be determined.
While the invention is susceptible to various modification and alternative forms, certain illustrative embodiments thereof have been shown by way of example in the drawing and will herein be described in detail.